Hydrogeology of Jimha Area

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Addis Ababa University


The studied area is located 335 Km. southwest of Addis Ababa in the southwestern Ethiopian plateau . Over half of the map-area is underlain by thick basalt flows inter layered with tuffs and volcanoclastic sediments . The Jimma Volcanics, which is mainly composed of massive rhyolite in thick flows alternating wjth trachytes, tuffs, ignimbrites and subordinate basalts, underlain small area in the eastern and northeastern part of the basin. Based on the hydrometeorologic data collected for the last 38 years an attempt has been made to calculate the hydrological parameters l i ke precipitation, evapotranspiration, and runoff. The ann ual mean rainfall in the basin is 1476.89 mm . The values of the annual potential evaportanspirtion and actual evapotranspiration are 837.51 mm and 822.29 mm respectively. The only runoff that leaves the studied area comes through Boye river and is 90 .77 million cubic meter of water. The amount of water which is actually available to recharge the groundwater circulation within the hydrogeological basin is 51.14 million cubic meter . The hydrogeological character of the outcropping lithotypes was discussed with particular reference to infiltration and their water bearing capacity. The hydr ogeology of the area varies in different parts of the basin. There is a great potential of groundwater in the valleys which is partly filled with alluvial sediments while aless significant ameunt exist within the velcanic recks ferming the uplifted berders ef the valleys. Determinatien ef the hydraulic parameters like transmissivity and permeability have been carried eut. Due to. the absence ef ebservatien well nearby a discharging wells it was impessible to. calculate the ceeff icient ef sterage, a significant earameter which measures the velume ef water the aquifer releases er take into. sterage. Fer basaltic aquifers a transmissivity value ef 18.39 m2/day and a permeability value ef 3.31 x 10-1 cm/ sec. have been calculated. A transmissivity value ef 1.89 m2 /day . and a permeability value ef 1.62 x 10-5 cm / sec. have been co.mputed fer beth alluvial and rhyelitic aquifers. Because ef the lack ef bere wells in the seuthwest, seuth, and seutheast lewer mest part ef the basin a thereugh study ef the actual variatien ef greundwater level was net pessible. Fer the ether part ef the basin, apprepriate greundwater level centeur lines was censtructed on the basis of the available static level data. Preper chemical analysis ef beth the greundwaters and surfaces water carried eut in the basin shew lew tetal disselved selids. All of the samples have less than 1000 mg / l disselved selids. The analysed samples also. cempared with the acceptable standard for human, agr icul tural, and industr ial use.